A conventional metal tube is formed by the steps of preparing a metal tape coated with rust-resisting oil, removing the rust-resisting oil from the metal tape with use of organic solvent or surface-active agent (surfactant), forming the metal tape to be round, and welding a butt portion of the rounded metal tape to provide a metal tube by means of a TIG welding torch.
In the process as described above, however, the rust-resisting oil will be burnt to deteriorate the quality of the metal tube, unless the rust-resisting oil is removed prior to the formation of the metal tube.
These days, the use of rinsing agent such as organic solvent, etc. has been requested to be abolished, so that the global environment will be protected from contamination.
In this circumstance, the inventors have repeated experiments, in which metal tapes having various anti-corrosion layers are prepared, to test forming properties in manufacturing metal tubes from the prepared metal tapes, and to check as to whether or not the metal tapes are damaged on the surfaces thereof due to the lack of the forming properties.